The term “grease” as used herein refers to a semisolid lubricant.
It is estimated that 90% of all machine bearings are lubricated by grease. While oil analysis is a widespread tool for monitoring bearing and lubricated component health of important oil lubricated equipment, grease analysis is not generally adopted in a similar manner for important grease lubricated machines. Analyzing a sample of grease taken from a grease-lubricated bearing sometimes is performed to pinpoint the cause of bearing problems in failure analysis. However, the ability to analyze grease has been historically limited by the inability to easily obtain representative samples of the grease in a sufficient quantity to perform cost-effective and meaningful analysis.
Wurzbach U.S. Pat. No. 7,984,661 discloses a method of obtaining a sample of grease from a machine and into a container for later analysis. The Wurzbach patent is incorporated by reference herein.
A known method for analyzing grease is to extrude a sample of the grease having a fixed, predetermined thickness from the container through a fixed die and onto a substrate, and exposing the fixed layer of grease to a light source. The light passes through the grease and through the substrate, and the light transmitted through the grease and substrate is detected by a detector. The spectrum of the light reaching the detector is compared to the spectrum obtained from a reference sample of grease. The differences in the spectrums relates to changes in the state or condition of the grease.
A problem with the known method for grease analysis is that grease has a tendency to “go dark” and substantially darken over the operating lifetime of the grease. The grease may go dark because of contamination or other causes indicating a bearing or machine problem, or the grease may go dark merely because of aging of the grease over the operating lifetime of the grease with normal operation of the bearing or machine (and without darkening caused by contamination or any other undesirable cause of grease change indicating a bearing or machine problem).
Grease that has gone dark may not transmit enough light to through the grease to be detected by the sensor. Or there may be some light transmitted through the grease but not enough light is detected to enable accurate determination of the color or color spectrum of the light passing through the grease. In either case the detector may report the color as being black or so close to black that meaningful comparison to a base color cannot be made.
Increasing the intensity of the light to increase light transmittal through the darkened grease does not solve the problem of analyzing grease that has gone dark.
Experiments with increasing light intensity found that useful results were often not obtained. It was theorized after the method disclosed herein was developed that the increased light intensity increases scattering effects. The scattering and the attenuation of light due to particulates or other constituents decreases the signal-to-noise ratio significantly to render the result indistinguishable for the purposes of making a meaningful measurement.